We are interested in defining the detailed molecular mechanisms of morphological transformation by human herpesviruses. As a first step, specific isolated DNA fragments representing 5 to 10% of the herpes simplex virus type 1 and 2 genomes were shown to consistently initiate focus formation in monolayers of contact inhibited mouse cells. In some cases cell lines derived from these foci contain stably integrated fragments of viral DNA but at less than one copy per cell. Other cell lines containing portions of the mtr-I and mtr-II sequences have been established by linked co-selection with the HSV thymidine kinase gene. Physical maps of sets of cloned DNA fragments across these transforming regions have been completed and we are in the process of constructing defined biochemical deletions and insertions in these plasmids with the hope of being able to obtain transformation defective host-range mutants. We are also investigating the possibility that HSV infection activates the expression of cellular transformation antigens and may also amplify certain middle repetitive cellular DNA sequences which have homology with specific sites in the HSV genome.